The instant invention relates to a method of preventing metal loss due to oxidation in molten aluminum and magnesium alloys. Specifically, covering the molten alloy surface with a layer of petroleum coke, which subsequently oxidizes at high temperatures to form a layer of carbon dioxide that insulates the alloy melt from ambient air oxygen, significantly reduces metal loss from the molten alloys due to oxidation.
Many metals and metal alloys, used especially in the foundry industries, suffer from loss of metal due to oxidation while in the molten state (xe2x80x9cmeltxe2x80x9d for brevity). This is particularly true for many aluminum-magnesium alloys. Significant and costly magnesium weight loss in the melt can be experienced. This leads to difficulty in controlling alloy composition, which is required to meet mechanical property specifications of the solidified alloy. As magnesium in the melt is depleted by loss to oxidation, magnesium concentration in the melt needs to be monitored and restored to desired levels by new magnesium additions.
Further, the significant metal weight loss in the melt due to oxidation results in the formation of dross on the surface of the melt, and oxide inclusions in the melt. Dross, as used herein, refers to undesirable oxides that form on the surface of a melt. Prior to casting the alloy, dross must be skimmed off of the melt surface, and disposed of in landfills. Oxide inclusions result in degradation of mechanical properties, such as ultimate tensile and yield strengths, of the alloys.
Previous methods that are employed to limit metal melt oxidation attempted to either blanket the melt in an atmosphere of inert gas, thus preventing oxidation of the melt surface, or to cover the melt surface with a solid inert barrier material that could physically prevent oxygen in the furnace atmosphere from reacting with the melt surface.
U.S. Pat. No. 6,024,779 describes a means to minimize oxidation of copper melts. Carbon sand, consisting of coke particles, is layered on top of a copper melt in thicknesses from about 1 millimeter to about 4 inches to prevent oxidation of the melt. The carbon sand forms a protective layer over copper and copper alloy melts and can be re-used. U.S. Pat. No. 6,485,541 teaches a method to reduce oxidation of aluminum melts. Inert refractory material is allowed to float on the melt, thus isolating the melt from oxygen in the air. U.S. Pat. No. 5,415,220 teaches the use of a salt covering for DC casting aluminum-lithium alloys. A mixture of lithium chloride and potassium chloride is preferred. The need for an inert atmosphere is obviated by use of the salt cover. U.S. Pat. No. 5,421,856 discloses the use of gas injectors in the melt that utilize inert gases to remove oxygen from the atmosphere above the melt.
Using an inert atmosphere to minimize oxidation requires specialized furnaces that seek to maintain an inert atmosphere directly above the melt surface. Such furnaces are costly to install, to maintain, and to use. However, the exclusion of practically all of the oxygen from the furnace atmosphere provides for low oxide formation.
Placing physical barriers on the melt surface allows for the use of ordinary furnaces that operate in ambient air. However, because of the high thermodynamic driving force for the reaction of certain metals, such as aluminum and magnesium, with oxygen, it is difficult to eliminate oxide formation simply by using physical barriers.
Accordingly, it would be advantageous to have an economical process that would combine the efficiency of inert atmosphere with the economy and the facility of using a physical barrier.
In addition to magnesium metal loss due to oxidation and general dross formation, it is well known in the aluminum alloy casting art that melt surface oxidation can result in various surface imperfections in cast ingots such as pits, vertical folds, oxide patches and the like, which can develop into cracks during casting or in later processing. A crack in an ingot or slab propagates during subsequent rolling, for example, leading to expensive remedial rework or scrapping of the cracked material.
The casting of alloys may be done by any number of methods known to those skilled in the art, such as direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC), hot top casting, continuous casting, semi-continuous casting, die casting, roll casting and sand casting. Each of these casting methods has a set of its own inherent problems, but with each technique, surface imperfections can still be an issue. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping involves the machining off a surface layer along the sides of an ingot after it has solidified.
Certain alloys, such as 7050 and other 7xxx alloys as well as 5182 and 5083 are especially prone to surface defects and cracking. In the past, beryllium has been added, usually at part per million (ppm) levels to some of these alloys to control melt surface defects, and to prevent magnesium loss due to oxidation. However, beryllium has been banned from aluminum products used for food and beverage packaging. Further, there have been increased concerns over the health risks associated with factory workers using beryllium and products containing beryllium. For this reason, although beryllium is effective at controlling surface defects and melt surface oxidation in aluminum cast ingots, a suitable replacement is needed.
U.S. Pat. No. 5,469,911 to Parker discloses a method for improving the surface quality of electromagnetically cast aluminum alloy ingots, which includes the addition of 0.01 to 0.04 wt. % calcium prior to the ingot head of an ingot mold. These levels of calcium are significantly higher than the ppm levels employed with beryllium. Such high levels of calcium can adversely affect the properties of the alloy.
U.S. Pat. No. 4,377,425 to Otani et al. discloses using calcium in high iron containing direct chill cast aluminum alloy ingots to minimize the occurrence of dendritic or so-called xe2x80x9cfir treexe2x80x9d crystal structures with a grain size of less than 150 microns. This method was particularly useful for AA1000 and AA5000 series aluminum alloys. The effect, if any, of calcium on the surface quality of the resulting ingots was not disclosed by Otani et al.
There remains a need for an effective alternative to beryllium to prevent surface imperfections, such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting, and to prevent melt surface oxidation. Such a method would be instrumental in preventing cracks, which can form during casting or can develop in later processing. Finally, the method preferably would have no adverse affect on alloy properties.
The primary object of the present invention is to provide an economical and efficient method for minimizing metal loss due to oxide formation on an aluminum alloy or magnesium alloy melt surface in furnaces operating in ambient air.
Another object of the present invention is to provide an in situ method of generating an inert atmosphere above an aluminum alloy or magnesium alloy melt surface without the requirement of specialized furnaces that are adapted to displace oxygen during operation.
A further object of the present invention is to provide a method of minimizing oxide formation on an aluminum alloy or magnesium alloy melt surface with the method having the efficiency of using an inert atmosphere and the facility of using a physical barrier placed on the melt surface.
Yet another object of the present invention is to provide a method to minimize dross formation on an aluminum alloy or magnesium alloy melt surface.
Still another object of the present invention is to provide a method to minimize oxide inclusions in an aluminum alloy or magnesium alloy solid, thereby maximizing the mechanical properties of that solid.
Still a further objective to this invention is to provide an effective alternative to beryllium alloying additions to aluminum alloys or magnesium alloys for the prevention of surface imperfections such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting.
These and other objects and advantages are met or exceeded by the instant invention, and will become more fully understood and appreciated with reference to the following description.
The instant invention provides a combination of a physical barrier and an inert atmosphere, which is effective in minimizing oxide formation in ordinary furnaces. As a result, dross formation is minimized and mechanical properties of the resultant metallic solid product are maximized. The method is intended for use in ordinary furnaces, which contain ambient air, and require no special gas handling equipment.
In a preferred embodiment, petroleum coke, also known in the art as carbon flake, is added to a melt surface to completely cover the melt surface with an initial layer thickness of about 0.125 inch to about 5 inch. In a more preferred embodiment the petroleum coke initial layer thickness is about 0.25 inch to about 3 inch. In a most preferred embodiment the petroleum coke initial layer thickness is about 0.5 inch to about 1 inch. At typical melt temperatures, in furnaces that are open to ambient air, the petroleum coke layer will immediately begin to oxidize with oxygen in the furnace air to form, in situ, an inert atmosphere of carbon dioxide gas, with the partial pressure of the carbon dioxide gas being highest at the petroleum coke/air interface. Diffusion of oxygen molecules from the furnace air through the high partial pressure of carbon dioxide at the petroleum coke/air interface becomes arduous. Oxygen that does diffuse through the in situ generated carbon dioxide layer is further inhibited from reaching the melt by being consumed in the reaction with the petroleum coke. An equilibrium will be established that governs the rate of petroleum coke consumption and in-situ carbon dioxide generation.
Conceptually, at the point where the reaction of petroleum coke with oxygen to form carbon dioxide gas reaches equilibrium, the petroleum coke layer is now referred to as a working layer. Practically, the working layer is attained immediately after adding the initial layer of petroleum coke. The petroleum coke working layer is monitored periodically. In a preferred embodiment the working layer thickness is maintained in the range of about 0.125 inch to about 5 inch by adding petroleum coke. In a more preferred embodiment the petroleum coke working layer thickness is maintained in the range of about 0.25 inch to about 3 inch by adding petroleum coke. In a most preferred embodiment the petroleum coke working layer thickness is maintained in the range of about 0.5 inch to about 1 inch by adding petroleum coke. Preferably, the petroleum coke layer is checked and maintained about every 6 to 12 hours. More preferably the petroleum coke layer is checked about ever 8 to 10 hours.
Maintaining a working layer of petroleum coke is important for the instant invention in that it is not only required for continued generation of a high partial pressure of inert carbon dioxide that protects the melt from oxidation, but also for providing a physical barrier that blocks the diffusion of any unreacted oxygen through the working petroleum coke layer.
If the molten metal is transferred from the furnace to a crucible, the working layer of petroleum coke does not need to be removed prior to metal transfer. The petroleum coke can remain on the melt surface through rotor or lance crucible degassing, if applicable.
For cast pouring, whether from a dip well of from a crucible, the working layer of petroleum coke must be skimmed from the melt surface prior to ladling the casting metal.